Despite all the 'criticism' on this thread, i don't find the idea half bad (I felt like having the forgotten king as the final boss for the ruins was somewhat lazy, credit to food for making the dungeon by the way, love the design but no new content or new enemy's, just the same ones all in a single dungeon, can't be hard to add new content can it? Doesn't give that much uniqueness to the dungeon as a whole, but then again, most bosses in the game are here anyway, so who am i to complain, idk). Also ruins is kinda like a full dungeon based version of pandora, with MORE types of bosses in a single dungeon compared to pandora, which is a bit 'dull' to me, still a great challenge though. Maybe its just me.

Maybe you could improve how this soul idea works, maybe when a certain amount of people die during the ruins, the final boss (this one) affects his damage, or makes the boss fight harder at the end. In other words, the more people that die while progressing through the dungeon, potentially the harder this boss gets, adding more shots or damage or more phases. Something to think about maybe.
Otherwise +1